This invention relates to an improvement in the apparatus and method for monitoring surfaces of the body disclosed in copending applications Ser. Nos. 509,039 and 337,304, which are hereby incorporated by reference. The invention particularly relates to preprocessing of images as described on pages 26-29 of Ser. No. 509,039 and on pages 22-24 of Ser. No. 337,304, U.S. Pat. No. 5,016,173 and more particularly to color transforms of images as described therein.